In medical or dental technology a treatment of the human body or the body of an animal or artificial parts thereof (prostheses) with a treatment instrument can be effected in various ways. In many cases only a treatment of the body is required, without changing its shape. It may be a question, for example, of a surface treatment which, for example, can be carried out with a treatment instrument having a special tool for the surface treatment, in which connection it may be a question, for example, of a massage treatment or an application of a therapeutic medium to the body. A different type of treatment consists, for example, in changing the shape of the body, such as is the case, for example, with processing involving removal of material by cutting, in the course of which, for example, a cut is made in the body tissue. Cutting treatments also include the preparation of a cavity with a cutting tool, or the surface treatment of a tooth in order to remove a coating, for example. Tools used for cutting may be, for example, rotary tools such as drills and milling cutters, files and those tools having at least one working surface that is abrasive in two dimensions--ie, that is studded with a plurality of cutting edges or cutting points arranged alongside one another, the tool being driven in oscillating manner by a vibrating drive. Such a treatment instrument or tool is described, for example, in WO 96/14024.
A great many demands are made of a treatment instrument of the present type. One requirement is that the treatment instrument should be of small or slim structural design, in order that the view of the treatment site is not impaired or is only negligibly impaired. Inasmuch as several different treatment tools are present in the case of a treatment instrument of the present type, the retaining device should enable an easy-to-handle and rapid exchange of the tools, so that the treatment instrument can be employed with appropriate power for different types of treatment. In the case of a treatment instrument that is known from WO 96/14024, for the purpose of retaining the tool on a protruding shank of the treatment instrument a transverse hole is provided in the shank, into which the tool shank is capable of being inserted and is capable of being locked to prevent it from slipping out by means of a fixing element which is capable of being displaced between a locking position set into a device for locking the tool shank and a release position releasing the tool shank and is biased into its locking position by the force of a spring. In the stated printed publication several embodiments of a retaining device for a tool are described. Inter alia it is possible to lock the tool so that it is capable of rotating freely about its longitudinal axis or capable of being immobilised in the retaining device in different rotary positions. In the case of a treatment instrument of the present type that can be inferred from the previously known DE 44 08 574 A1, for the purpose of locking the tool in the retaining device of the treatment instrument a plurality of spheres are arranged in distributed manner on the periphery of the retaining device which are capable of being pressed radially inwards by means of an inner conical surface of an axially displaceable actuating element into a locking position in which they are set into spherical indentations of the tool shank and, after a preceding axial displacement of the actuating part in the course of withdrawing the tool shank from the retaining device, are capable of being driven radially outwards into a release position. It has also already been proposed to configure a retaining device in such a way that in the course of axial displacement of the tool, for example in order to introduce it into the retaining device or to withdraw it therefrom, the locking parts are capable of being automatically driven into their release position releasing the tool shank.